Ferrite stainless steel for use in producing urea water tank

ABSTRACT

This ferrite stainless steel for use in producing a urea water tank includes: in terms of mass %, C: 0.05% or less; N: 0.05% or less; Si: 0.02 to 1.5%; Mn: 0.02 to 2%; Cr: 15 to 23%; and either one or both of Nb and Ti at a content within a range of 8(C+N) to 1% (herein, C and N represent the contents of C and N (expressed by mass %), respectively, and the numerical values shown in front of the atomic symbols represent constant numbers), with the remainder being iron and unavoidable impurities, wherein an effective amount of Cr expressed by any one of the following Equations (I), (II), and (III) is 15% or more (herein, the atomic symbols in Equations (I) to (III) represent the contents of the elements (expressed by mass %), and the numerical values shown in front of the atomic symbols represent constant numbers). Here, the effective amount of Cr=Cr+4Si−2Mn in the case where only Nb is contained, the effective amount of Cr=Cr+4Si−2Mn−10Ti in the case where only Ti is contained, and the effective amount of Cr=Cr+4Si−2Mn−(10Ti−3Nb) in the case where both of Nb and Ti are contained.

TECHNICAL FIELD

The present invention relates to a ferrite stainless steel being used for a device that reduces NO_(x) from exhaust gas by using a urea aqueous solution (urea water) in an internal combustion engine, mainly in a diesel engine, and, in particular, for equipments in a urea-Selective Catalytic Reduction (SCR) system for vehicles and the like, specifically, for a urea water tank that is utilized when storing, producing, and transporting urea water.

The present application claims priority on Japanese Patent Application No. 2008-190065, filed on Jul. 23, 2008, the content of which is incorporated herein by reference.

BACKGROUND ART

In recent years, as a result of increased concern about environmental issues, exhaust gas emission regulations are becoming stricter, and major efforts are being made to suppress carbon dioxide emission. In the automotive industry, in addition to efforts made to provide fuels such as bio-ethanol and biodiesel fuel, a variety of efforts are being made to improve fuel efficiency by reducing automobile weight or installing a heat exchanger that recovers exhaust heat, and to install an exhaust gas treatment device such as an exhaust gas recirculation (EGR), a diesel particulate filter (DPF), a urea SCR system, and the like.

Among the exhaust gas treatment devices, the urea SCR system is one of the NO_(x) reducing systems and in which urea water is used as a NO_(x) reducing agent. Compared with the case where liquid ammonia or ammonia water is used as a reducing agent, urea water has an advantage of being safe and relatively easy to treat; and as a result, it is being examined for application to stationary NO_(x) reducing systems for distributed power-supply facilities installed in urban areas and the like, as well as automobiles.

In the urea SCR system, the urea water sprayed into the exhaust gas is decomposed by heat and moisture so as to produce ammonia. Then, the ammonia and NO_(x) are selectively reduced on a catalyst so as to be decomposed into innocuous nitrogen.

The urea water used in the above-described case is a urea aqueous solution (urea water) having a high concentration of 25 to 45%. In general, in the urea SCR system for vehicles, a urea aqueous solution having a concentration of about 32.5% and the lowest freezing point is used, which is prescribed in “NO_(x) reduction additive in diesel engines—AUS 32-Part 1: Properties” by JIS K2247-1 (The Automotive Standards JASO E502 is also a similar standard). The standards also strictly prescribe the concentration of impurity elements, and elements in relation to stainless steel are prescribed to fulfill Fe: less than 0.5, Cr: less than 0.2, Ni: less than 0.2, Cu: less than 0.2 (all in the units of mg/kg).

Any material being used for a urea water tank needs to have extremely high corrosion resistance. Because it is not permitted that the concentration of impurities in the urea water exceeds the range prescribed in the above-mentioned regulations due to elution from materials used in equipments for storing, producing, and transporting the urea water.

In addition, since the tank is normally used outside, as in automobiles, and for a long time period of ten years or more, there is a concern that the tank is penetrated by rainwater, sea-salt particles and the like, which may lead to leakage of the urea water in the tank. Since leakage of the urea water may cause a deterioration in function of the NO_(x) reducing system, this needs to be avoided. Therefore, any material being used for the urea water tank needs to have an excellent corrosion resistance against salt damage on the outside surface.

Patent Document 1 discloses a supply device of high grade urea water and a method for supplying high grade urea water using the same. Patent Document 1 discloses a supply device which includes: an electromotive pump having a high grade urea water supply port equipped with an air-removing mechanism and an exhaust hose equipped with a gun nozzle; and a high-density polyethylene intermediate bulk container (IBC) tank having a net volume of 1200 to 1500 L. It is also disclosed that the electromotive pump is preferably made of reinforced plastic and the pump shaft is preferably made of one of stainless alloy (SUS304), Hastelloy, and Inconel alloy. SUS304 refers to austenite stainless steel, and in Patent Document 1, there is not any direct description regarding ferrite stainless steel.

Patent Document 2 discloses two-phase stainless steel for a urea-producing plant, welding materials, a urea-producing plant and equipment thereof. Patent Document 2 discloses two-phase stainless steel containing Cr: 26% or more and less than 28%, Ni: 6 to 10%, Mo: 0.2 to 1.7%, and W: more than 2% and 3% or less. Urea is synthesized from ammonia and carbon dioxide gas under high temperatures and high pressures. Urea has highly corrosive nature due to the existence of intermediate products of the synthesis reaction such as ammonium carbamate and the like. Therefore, it is necessary to use materials that can endure corrosion wastage so as to prevent internal substances from being leaked.

Compared with the corrosive environments under high temperatures and high pressures in the urea-synthesizing plants, environments of a high-concentration urea water which is used near room temperature in the urea SCR system and the like are mild because of the lower temperature and the absence of the intermediate products of the synthesis reaction. However, it is necessary to suppress elution of stainless steel elements such as Fe, Cr, Ni, Cu, and the like so as to fulfill the above-described JIS standard; and therefore, the material needs to have an excellent corrosion resistance on the inside surface for elution suppression and the like. In addition, the material needs to have a corrosion resistance against salt damage induced by rainwater, sea-salt particles, and the like on the outside surface.

Patent Document 3 discloses ferrite stainless steel having excellent brazeability. It is disclosed that the ferrite stainless steel is suitable for members having complicated shapes and produced by brazed welding, such as a urea water tank or the like being used for a urea SCR system for vehicles.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Unexamined Patent Application, First     Publication No. 2007-113484 -   Patent Document 2: Japanese Unexamined Patent Application, First     Publication No. 2003-301241 -   Patent Document 3: PCT International Publication No. WO2009/084526

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention aims to provide a ferrite stainless steel suitable for a device that reduces NO_(x) from exhaust gas by using urea water in an internal combustion engine, mainly in a diesel engine, and, in particular, for equipments in a urea SCR system for vehicles and the like, specifically, a urea water tank that is utilized when storing, producing, and transporting urea water. An elution amount of constituent elements eluted from the ferrite stainless steel into a high-concentration urea aqueous a solution (urea water) is small, and the ferrite stainless steel has an excellent corrosion resistance against salt damage.

Means for Solving the Problems

As a result of dedicated studies to solve the above problem, the inventors of the present invention have found that it is important to form a passive film containing Cr in the surface of a steel in order to reduce an elution amount of the constituent elements of the steel into a urea aqueous solution having a high concentration of 25 to 45% on the inside surface and to attain an excellent corrosion resistance against salt damage on the outside surface, and consequently, it is necessary to contain an appropriate amount of Cr.

It is well known that the corrosion resistance of the steel is improved by forming a passive film containing Cr on the steel surface. However, for instance, in a urea water tank used for a urea SCR system for vehicles, there is a concern that the passive film formed on the steel used for the urea water tank may elute or the steel located below the passive film may elute, at the instant when the tank in a normal pre-use state of being exposed to air is exposed to a high-concentration urea aqueous solution.

The inventors of the present invention have found that a ferrite stainless steel containing 10% or more of Cr can form a uniform passive film that can suppress an elution of the passive film formed on the surface of the steel and an elution of the steel located below the passive film via the passive film in the urea water having a high concentration of 25 to 45% used in the urea water tank (Japanese Patent Application No. 2008-62598).

Meanwhile, in terms of the corrosion resistance against salt damage, the forming of the uniform passive film capable of suppressing the elution is also important to enhance the resistance against chloride ions contained in sea-salt particles and the like; and thereby, the occurrence of the corrosion is suppressed. In the outdoor environment, since wet and dry cycles are repeated, the repeated wet and dry cycles make the amount of the chloride ions concentrated. This results in a high-concentration chloride ion environment, which is more corrosive than the high-concentration urea aqueous solution environment. Therefore, a larger amount of Cr is required to form a uniform and stable passive film. In the present invention, it has been found that the lower limit of the Cr amount should be 15%.

The urea water tank which is the subject of the present invention is normally joined and assembled by welding or brazing. There are some cases where an oxide film is formed on the surface of a steel in the welded (or brazed) portions. Even in the case where the oxide film is formed, it is still necessary to suppress an elution of the constituent elements from the steel into a high-concentration urea water on the inside surface and to suppress a corrosion due to the salt damage on the outside surface. Since the diffusion rate of Cr in a ferrite stainless steel is greater than that in an austenite stainless steel, a lack of Cr just below the oxide film can be suppressed. Since it is important to retain a large amount of Cr just below the oxide film so as to suppress an elution of constituent elements from the steel having the oxide film into a high-concentration urea aqueous solution, the amount of Cr necessary to suppress the elution from the welded (or brazed) portions in the ferrite stainless steel can be made smaller than that in the austenite stainless steel.

Furthermore, from the dedicated studies, the inventors of the present invention have found that the effective amount of Cr as expressed by any one of the following Equations (I), (II), and (II) needs to be 10% or more so as to secure the amount of Cr just below the oxide film and to fulfill the regulation in relation to the elution of the constituent elements into a high-concentration urea water (Fe: <0.5, Cr: <0.2, Ni: <0.2, Cu: <0.2 (all in the units of mg/kg)) in the case where the oxide film is formed (herein, the atomic symbols in Equations (I) to (III) represent the contents of the elements (expressed by mass %), and the numerical values shown in front of the atomic symbols represent constant numbers). In addition, it has been found that the effective amount of Cr needs to be 15% or more so as to suppress a corrosion due to the salt damage that is severer than the high-concentration urea water.

In the case where only Nb is contained,

the effective amount of Cr=Cr+4Si−2Mn  (I)

In the case where only Ti is contained,

the effective amount of Cr=Cr+4Si−2Mn−10Ti  (II)

In the case where Nb and Ti are contained,

the effective amount of Cr=Cr+4Si−2Mn−(10Ti−3Nb)  (III)

Equations (I) to (III) are alloy element indices where an influence of Si, Mn, Ti, and Nb contained in the steel on an effect of improving corrosion resistance due to Cr is taken into consideration, and are utilized for calculating a numerical value as an index of an effective amount of Cr that contributes to the improvement of the corrosion resistance of the steel.

Although the effects of Si, Mn, Ti, and Nb included in Equations (I) to (III) are not fully understood, the effects of the respective elements are considered as follows:

Si is a useful element that forms an oxide just below chromium oxide so as to suppress the oxidation of Cr. Mn accelerates the generation of a spinel type oxide containing Cr and Mn so as to reduce the effective amount of Cr. Ti remarkably accelerates the growth of Cr oxide so as to considerably reduce the effective amount of Cr. Nb has an effect to reduce the effect of Ti of accelerating the growth of chromium oxide so as to suppress the decrease in the effective amount of Cr due to Ti.

In addition, in the case where a brazed jointing is conducted to assemble a urea water tank, brazeability relative to brazing metals of Ni and Cu is demanded. As a result of dedicated studies on the effect of alloy elements on brazeability, the inventors of the present invention have found that there are maximum values in the content of Ti which is frequently added to improve the formability or intergranular corrosion property in a ferrite stainless steel, and the content of Al which is added for deoxidation, in order to secure a satisfactory brazeability as shown in the following Equations (IV) and (V) (herein, the atomic symbols in Equations (IV) and (V) represent the contents of the elements (expressed by mass %), and the numerical values shown in front of the atomic symbols represent constant numbers).

Ti−3N≦0.03  (IV)

10(Ti−3N)+Al≦0.5  (V)

In order to obtain a satisfactory brazeability, molten brazing metal needs to adhere and spread out on the surface of a stainless steel. The wettability of brazing metal is affected by a surface film formed on the stainless steel in a brazing atmosphere.

In the brazing atmosphere, even in the case where conditions allowing the reduction of iron and chromium oxides can be maintained, Ti and Al, which are oxidized more easily than Fe and Cr, form oxides so as to hinder the adhering and spreading out of the brazing metal; and thereby, the brazeability is degraded. Ti and Al solid solutions contribute to a formation of such an oxide film. If the Ti and Al solid solutions exist as relatively stable nitrides even at the brazing temperature, the Ti and Al solid solutions do not contribute to the film formation; and therefore, the Ti and Al solid solutions do not hinder the adhering and spreading out of the brazing metal. From these viewpoints, the relationship between the contents of Ti and Al and the adhering-and-spreading-out property (wettability) of the brazing metal has been studied.

As a result, as shown in the examples described later, it has been confirmed that the adhering-and-spreading-out property becomes satisfactory in the case where the conditions of Ti−3N≦0.03, Al≦0.5%, and 10(Ti−3N)+Al≦0.5 are fulfilled. With regard to steels of which the contents of Ti and Al did not fulfill the above-described conditions, the surface films after the thermal treatment of brazing were analyzed. As a result, it was revealed that oxide films including concentrated Ti and Al were uniformly formed within a thickness of several tens of nanometers to several hundreds of nanometers. It is considered that such a film formation hinders the adhering-and-spreading-out property of the brazing metal.

Furthermore, since the urea water tank which is the subject of the present invention needs to have a strength, it is desirable to suppress the decrease in the strength after brazing. In the case where brazing is conducted at high temperatures within a range of 1000 to 1100° C. such as Ni brazing and Cu brazing, it has been considered that it is important to suppress the decrease in the strength induced by grain coarsening.

Pinning effect due to precipitates is useful to suppress the grain coarsening, and the inventors of the present invention have found that the amount of precipitates and the stability of carbonitrides that are useful for the suppression of grain coarsening can be secured by utilizing Ti and Nb carbonitrides as the precipitates and by setting the total amount of C and N (expressed by mass %) to be in a range of 0.015% or more (Japanese Patent Application No. 2007-339732).

The present invention aims to provide a ferrite stainless steel for use in producing a urea water tank which has an improved corrosion resistance against salt damage together with the properties described in the previous two Japanese Patent Applications. That is, the present invention aims to provide the ferrite stainless steel showing a small degree of elution of constituent elements into a high-concentration urea water and an excellent corrosion resistance against salt damage. The summary of the present invention is as follows as described in the claims:

(1) A ferrite stainless steel for use in producing a urea water tank includes: in terms of mass %, C: 0.05% or less; N: 0.05% or less; Si: 0.02 to 1.5%; Mn: 0.02 to 2%; Cr: 15 to 23%; and either one or both of Nb and Ti at a content within a range of 8(C+N) to 1% (herein, C and N represent the contents of C and N (expressed by mass %), respectively, and the numerical values shown in front of the atomic symbols represent constant numbers); with the remainder being iron and unavoidable impurities, wherein an effective amount of Cr expressed by any one of the following Equations (I) to (III) is 15% or more (herein, the atomic symbols in Equations (I) to (III) represent the contents of the elements (expressed by mass %), respectively, and the numerical values shown in front of the atomic symbols represent constant numbers).

In the case where only Nb is contained,

the effective amount of Cr=Cr+4Si−2Mn  (I)

In the case where only Ti is contained,

the effective amount of Cr=Cr+4Si−2Mn−10Ti  (II)

In the case where both of Nb and Ti are contained,

the effective amount of Cr=Cr+4Si−2Mn−(10Ti−3Nb)  (III)

(2) The ferrite stainless steel for use in producing a urea water tank according to (1), which further includes, in terms of mass %, one or more selected from Mo: 3% or less, Ni: 3% or less, Cu: 3% or less, V: 3% or less, and W: 5% or less.

(3) The ferrite stainless steel for use in producing a urea water tank according to any one of (1) and (2), which further includes, in terms of mass %, one or more selected from Ca: 0.002% or less, Mg: 0.002% or less, and B: 0.005% or less.

(4) The ferrite stainless steel for use in producing a urea water tank according to any one of (1) to (3) wherein a content of C+N is in a range of 0.015% or more.

(5) The ferrite stainless steel for use in producing a urea water tank according to any one of (1) to (4), which further comprises, in terms of mass %, Al: 0.5% or less; and wherein Equations (IV) and (V) are fulfilled (herein, the atomic symbols in Equations (IV) and (V) represent the contents of the elements (expressed by mass %); and the numerical values shown in front of the atomic symbols represent constant numbers).

Ti−3N≦0.03  (IV)

10(Ti−3N)+Al≦0.5  (V)

Effects of the Invention

In accordance with the present invention, it is possible to provide a ferrite stainless steel showing a small degree of elution of constituent elements into a high-concentration urea water and an excellent corrosion resistance against salt damage. Therefore, it is possible to provide a preferred material used for a device that reduces NO_(x) from exhaust gas by using urea water in an internal combustion engine, mainly in a diesel engine, and, in particular, a device related to a urea SCR system for vehicles, and preferred for a tank being used when storing, producing, and transporting urea water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a figure showing the relationship between the adhering-and-spreading-out property of brazing metal and the amounts of Ti and Al.

FIG. 2 is a figure showing the relationship between the effective amount of Cr and the maximum corrosion depths in a cyclic corrosion test.

FIG. 3 is a figure showing the conditions of the cyclic corrosion test.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention has been made based on the above-described findings. Hereinafter, the chemical compositions defined by the present invention will be described in more detail.

C: Since C degrades intergranular corrosion resistance and formability, it is necessary to adjust the content of C to be at a low level. Therefore, the content of C is set to be in a range of 0.05% or less. However, since an excessively low content leads to the increase in refining cost, it is desirable to set the content of C to be in a range of 0.002% or more.

N: N is a useful element for pitting corrosion resistance; however, N degrades the intergranular corrosion resistance and the formability. Therefore, it is necessary to adjust the content of N to be at a low level. Accordingly, the content of N is set to be in a range of 0.05% or less. However, since an excessively low content leads to an increase in refining cost, it is desirable to set the content of N to be in a range of 0.002% or more.

Si: Si is useful as a deoxidization element, and is also an effective element for corrosion resistance; however, Si degrades the formability. Therefore, the content of Si is set to be in a range of 0.02 to 1.5%.

Mn: Mn is useful as a deoxidization element; however, Mn degrades corrosion resistance when an excessive content of Mn is included. Therefore, the content of Mn is set to be in a range of 0.02 to 2%.

Cr: Cr is the most important element in the present invention, and the content of Cr needs to be at least 15% or more so as to reduce an elution amount of constituent elements into high-concentration urea water and to attain an excellent corrosion resistance against salt damage. When the content of Cr increases, elution characteristics become more stable; however, the formability and the manufacturability deteriorate. Consequently, the upper limit of the content of Cr is set to 23% or less. The content of Cr is preferably in a range of 16% or more, and more preferably in a range of 18% or more.

Nb and Ti: A urea water tank which is the subject of the present invention is often jointed and assembled by welding or brazing. Nb and Ti are useful elements having the effects of fixing C and N and improving the intergranular corrosion resistance in welded (or brazed) portions. However, when an excessive content of Nb and Ti are included, Nb and Ti cause a negative effect on the formability and the manufacturability. Therefore, the content of either one or both of Nb and Ti is set to be in a range of 8(C+N) to 1%, and preferably in a range of 8(C+N) to 0.6% (herein, C and N represent the contents of C and N (expressed by mass %); respectively, and the numerical values shown in front of the atomic symbols represent constant numbers).

In the case where a brazed jointing is conducted to assemble a urea water tank, the content of Ti needs to be controlled to fulfill Ti−3N≦0.03 so as to secure a satisfactory brazeability (herein, the atomic symbols in the equation represent the contents of the elements (expressed by mass %), and the numerical values shown in front of the atomic symbols represent constant numbers). The value of Ti−3N is preferably in a range of 0.02% or less. However, since the formability deteriorates when the content of Ti is excessively low, it is preferable to adjust the content of Ti so as to fulfill the value of Ti−3N to be in a range of −0.08% or more. In the case where the formability and the like are not particularly required, Ti may not be included.

Effective amount of Cr: In the present invention, the effective amount of Cr expressed by any one of Equations (I), (II), and (III) is set to be in a range of 15% or more (herein, the atomic symbols in Equations (I) to (III) represent the contents of the elements (expressed by mass %); and the numerical values shown in front of the atomic symbols represent constant numbers).

In the case where only Nb is contained,

the effective amount of Cr=Cr+4Si−2Mn  (I)

In the case where only Ti is contained,

the effective amount of Cr=Cr+4Si−2Mn−10Ti  (II)

In the case where both of Nb and Ti are contained,

the effective amount of Cr=Cr+4Si−2Mn−(10Ti−3Nb)  (III)

It is necessary to set the effective amount of Cr calculated by the Equations (I) to (III) to be in a range of 10% or more so as to obtain a ferrite stainless steel showing a remarkably small degree of elution of constituent elements into high-concentration urea water and an excellent corrosion resistance that fulfills JIS K2247-1 by securing the amount of Cr just below an oxide film under conditions where the oxide film is formed in the steel surface, such as the case where the steel is subjected to welding or brazed jointing. In addition, the present invention demands the corrosion resistance against salt damage on the outside surface, and it is necessary to set the effective amount of Cr to be in a range of 15% or more, preferably in a range of 16% or more, and more preferably in a range of 18% or more so as to be compatible with the corrosion resistance in high-concentration urea water.

Mo: If necessary, it is possible to contain 3% or less of Mo so as to improve the corrosion resistance. The content of Mo needs to be 0.3% or more so as to obtain a stable effect. If an excessive content of Mo is included, Mo degrades the formability, and Mo leads to an increase in cost since Mo is expensive. Therefore, it is preferable to contain Mo at a content within a range of 0.3 to 3%.

Ni: If necessary, it is possible to contain 3% or less of Ni so as to improve the corrosion resistance. The content of Ni needs to be 0.2% or more so as to obtain a stable effect. If an excessive content of Ni is included, Ni degrades the formability, and Ni leads to an increase in cost since Ni is expensive. Therefore, it is preferable to contain Ni at a content within a range of 0.2 to 3%.

Cu: If necessary, it is possible to contain 3% or less of Cu so as to improve the corrosion resistance. The content of Cu needs to be 0.2% or more so as to obtain a stable effect. If an excessive content of Cu is included, Cu degrades the formability, and Cu leads to an increase in cost since Cu is expensive. Therefore, it is preferable to contain Cu at a content within a range of 0.2 to 3%.

V: If necessary, it is possible to contain 3% or less of V so as to improve the corrosion resistance. The content of V needs to be 0.2% or more so as to obtain a stable effect. If an excessive content of V is included, V degrades the formability, and V leads to an increase in cost since V is expensive. Therefore, it is preferable to contain V at a content within a range of 0.2 to 3%.

W: If necessary, it is possible to contain 5% or less of W so as to improve the corrosion resistance. The content of W needs to be 0.5% or more so as to obtain a stable effect. If an excessive content of W is included, W degrades the formability, and W leads to an increase in cost since W is expensive. Therefore, it is preferable to contain W at a content within a range of 0.5 to 5%.

Ca: Ca has a deoxidization effect and the like, and is a useful element for refining; and therefore, if necessary, Ca may be included at a content within a range of 0.002% or less. If Ca is contained, it is preferable to contain 0.0002% or more of Ca so as to obtain a stable effect.

Mg: Mg has a deoxidization effect and the like, and is a useful element for refining, and Mg also refines the microstructure and is useful for improving the formability and toughness. Therefore, if necessary, Mg may be included at a content within a range of 0.002% or less. If Mg is contained, it is preferable to contain 0.0002% or more of Mg so as to obtain a stable effect.

B: B is a useful element for improving the secondary formability. Therefore, if necessary, B may be included at a content within a range of 0.005% or less. If B is contained, it is preferable to contain 0.0002% or more of B so as to obtain a stable effect.

C+N: In the case where a brazed jointing is conducted to assemble a urea water tank, the content of C+N needs to be in a range of 0.015% or more, and preferably in a range of 0.02% or more so as to suppress a decrease in strength due to grain coarsening which occurs when being brazed. If an excessive content of C and N is included, C and N degrade the intergranular corrosion resistance and the formability. Therefore, it is preferable to set the upper limit of C+N to 0.04% or less.

Al: Al has a deoxidization effect and the like, and is a useful element for refining, and Al also has an effect of improving the formability. Therefore, if necessary, Al may be included. In the case where a brazed jointing is conducted to assemble a urea water tank, it is necessary to secure a satisfactory brazeability; and therefore, it is preferable to set the content of Al to be in a range of 0.5% or less.

In addition, in terms of the relationship with Ti that affects the brazeability similarly to Al, it is preferable to fulfill the equation of 10(Ti−3N)+Al≦0.5 so as to secure a satisfactory brazeability (herein, the atomic symbols in the equation represent the contents of the elements (expressed by mass %), and the numerical values shown in front of the atomic symbols represent constant numbers).

With regard to an unavoidable impurity of P, it is preferable to set the content of P to be in a range of 0.04% or less from the perspective of the weldability. In addition, with regard to S, it is preferable to set the content of S to be in a range of 0.01% or less from the perspective of the corrosion resistance.

As a method for manufacturing the stainless steel of the present invention, it is possible to apply a general method for manufacturing a ferrite stainless steel. In general, a molten steel is prepared in a converter or an electric furnace, and the molten steel is refined in an AOD furnace, a VOD furnace, or the like, and the refined molten steel is subjected to a continuous casting or an ingot-making method so as to obtain a slab, and then the slab is subjected to a process of hot rolling-annealing of a hot-rolled steel sheet-pickling-cold rolling-final annealing-pickling so as to manufacture a ferrite stainless steel. If necessity, the annealing of the hot-rolled steel sheet may be omitted, and the process of cold rolling-final annealing-pickling may be repeated.

EXAMPLES

With regard to cold-rolled steel sheets having the chemical compositions of Nos. 1 to 14 shown in Tables 1 and 2, adhering-and-spreading-out properties of brazing metal were evaluated Here, the column of Equation (IV) in Table 2 represents the value of Ti−3N, the column of Equation (V) in Table 2 represents the value of 10(Ti−3N)+Al.

(The Adhering-and-Spreading-Out Property of the Brazing Metal)

A test specimen having a width of 50 mm and a length of 70 mm was cut off from a cold rolled steel sheet, and one surface of the specimen was subjected to wet-polishing by emery paper down to 400-grit. Then, 0.1 g of Ni brazing alloy was placed on the polished surface, and the test specimen was heated at 1100° C. in a vacuum atmosphere of 5×10⁻³ torr (about 0.6666 Pa) for ten minutes.

After cooling down to room temperature, the area of the brazing metal after heating was measured. The brazeability was evaluated as good if the area of the brazing metal after heating is twice or more of the area of the brazing metal before heating, and the brazeability was evaluated as bad if the area of the brazing metal after heating is less than twice of the area of the brazing metal before heating.

TABLE 1 Chemical Composition (mass %) No. C Si Mn P S Cr Ti Nb Al N Other 1 0.012 0.42 0.15 0.028 0.0015 19.42 0.004 0.39 0.025 0.018 0.42Cu, 0.32Ni, 0.0010Ca 2 0.013 0.55 0.45 0.029 0.0008 16.58 0.002 0.55 0.004 0.015 0.32Ni, 0.35Cu 3 0.006 0.12 0.19 0.022 0.0010 18.84 0.004 0.42 0.036 0.010 1.86Mo, 0.0003B 4 0.016 0.25 0.18 0.029 0.0011 18.23 0.021 0.36 0.036 0.014 0.52Cu, 1.02Mo 5 0.007 0.16 0.15 0.022 0.0008 20.25 0.012 0.22 0.015 0.009 1.03Ni, 1.08Mo 6 0.014 0.33 0.45 0.030 0.0014 18.15 0.015 0.36 0.055 0.015 2.15W, 0.35V 7 0.015 0.40 0.32 0.025 0.0019 20.88 0.042 0.40 0.046 0.010 0.34Ni 8 0.016 0.41 0.29 0.024 0.0016 19.19 0.066 0.42 0.086 0.015 1.88W, 0.0005Mg 9 0.018 0.39 0.33 0.023 0.0015 19.34 0.032 0.39 0.35  0.009 0.56Ni, 0.38V, 0.0004Ca 10 0.008 0.18 0.15 0.026 0.0011 17.25 0.25 0.002 0.042 0.010 1.12Mo, 0.0005B 11 0.007 0.11 0.12 0.025 0.0012 18.85 0.12 0.22 0.056 0.012 1.80Mo, 0.0004B 12 0.012 0.33 0.25 0.025 0.0012 18.22 0.004 0.35 0.58  0.014 0.29Ni 13 0.010 0.42 0.36 0.026 0.0007 16.89 0.062 0.003 0.36  0.012 14 0.011 0.15 0.22 0.028 0.0009 19.12 0.073 0.25 0.041 0.008 1.90Mo

TABLE 2 Equation Equation C + N 8(C + N) Ti + Nb No. (IV) (V) (mass %) (mass %) (mass %) 1 −0.050  −0.48  0.03 0.24 0.394 2 −0.043  −0.43  0.028 0.224 0.552 3 −0.026  −0.22  0.016 0.128 0.424 4 −0.021  −0.17  0.03 0.24 0.381 5 −0.015  −0.14  0.016 0.128 0.232 6 −0.030  −0.25  0.029 0.232 0.375 7 0.012 0.17 0.025 0.2 0.442 8 0.021 0.30 0.031 0.248 0.486 9 0.005 0.40 0.027 0.216 0.422 10 0.220 2.24 0.018 0.144 0.252 11 0.084 0.90 0.019 0.152 0.34  12 −0.038  0.20 0.026 0.208 0.354 13 0.026 0.62 0.022 0.176 0.065 14 0.049 0.53 0.019 0.152 0.323

The results are shown in FIG. 1, and it has been confirmed that, among Nos. 1 to 14 shown in Tables 1 and 2, steels fulfilling the conditions of Ti−3N≦0.03, Al≦0.5%, 10(Ti−3N)+Al≦0.5 are evaluated as having good brazeability. Here, the underlined values in Tables 1 and 2 represent values which do not fulfill the above-described conditions.

Steels having chemical compositions shown in Tables 3 and 4 were melted and were subjected o a process of normal hot rolling, cold rolling, and annealing to manufacture steel sheets having a thickness of 1 mm. With regard to these cold rolled steel sheets, the corrosion resistance was evaluated by corrosion tests, and the brazeability was also evaluated. As the corrosion tests, an immersion test in a urea aqueous solution (urea water) was carried out for the inside surface and a cyclic corrosion test was carried out for the outside surface. The results are shown in Table 5 and FIG. 2.

Here, the effective amount of Cr column in Table 4 with the symbol of *1 represents the value of Cr+4Si−2Mn when containing only Nb, the value of Cr+4Si−2Mn−10Ti when containing only Ti, and the value of Cr+4Si−2Mn−(10Ti−3Nb) when containing both of Nb and Ti.

In addition, the column of Equation IV in Table 4 with the symbol of *2 represents the value of Ti−3N, and the column of Equation V with the symbol of *3 represents the value of 10(Ti−3N)+Al.

The underlined values in Tables 3 and 4 represent values outside the range of the present invention.

TABLE 3 Testing Example Chemical Composition (mass %) No. C N Si Mn P S Cu Ni Example of the Invention 1 0.014 0.018 0.51 0.38 0.025 0.0008 — — Example of the Invention 2 0.004 0.006 0.26 0.31 0.025 0.0008 — — Example of the Invention 3 0.009 0.012 0.94 0.88 0.021 0.0005 — — Example of the Invention 4 0.019 0.015 0.42 0.36 0.028 0.0011 — — Example of the Invention 5 0.006 0.011 0.45 0.15 0.026 0.0012 — — Example of the Invention 6 0.010 0.010 0.22 0.16 0.025 0.0008 0.33 — Example of the Invention 7 0.026 0.033 0.33 0.95 0.022 0.0015 — — Example of the Invention 8 0.016 0.016 0.42 0.40 0.026 0.0008 0.41 0.35 Example of the Invention 9 0.011 0.014 0.11 0.12 0.025 0.0010 — 2.86 Example of the Invention 10 0.008 0.009 0.48 0.22 0.021 0.0009 0.39 — Example of the Invention 11 0.005 0.012 0.25 0.21 0.028 0.0007 1.37 — Comparative Example 12 0.011 0.011 0.35 0.31 0.029 0.0007 — — Comparative Example 13 0.010 0.012 0.32 0.32 0.028 0.0022 — — Comparative Example 14 0.012 0.008 0.25 0.12 0.029 0.0019 — — Testing Example Chemical Composition (mass %) No. Cr Mo W V Ti Nb Al Ca Mg B Example of the Invention 1 16.38 — — — — 0.45 — — — — Example of the Invention 2 17.25 — — — 0.21 — — — — — Example of the Invention 3 15.23 — — — — 0.41 — — — — Example of the Invention 4 19.25 — — — — 0.45 — — — — Example of the Invention 5 21.35 — — — 0.12 0.23 — — — — Example of the Invention 6 19.52 1.92 — — — 0.48 — — — — Example of the Invention 7 15.96 — 1.98 0.45 — 0.75 — — — — Example of the Invention 8 19.42 — — — — 0.39 0.035 0.0008 — — Example of the Invention 9 21.05 0.98 — — — 0.38 0.025 0.0003 — 0.0004 Example of the Invention 10 21.78 0.82 — — — 0.41 0.012 — — — Example of the Invention 11 17.56 0.19 — — — 0.68 0.029 0.0007 0.0002 — Comparative Example 12  8.51 — — — 0.19 — 0.015 0.0003 0.0002 0.0005 Comparative Example 13 13.67 — — — — 0.42 — — — — Comparative Example 14 15.16 — — — 0.30 — — — — —

TABLE 4 Testing Effective Amount Example of Cr *¹ Equation iv *² Equation v *³ C + N 8(C + N) No. (mass %) (mass %) (mass %) (mass %) (mass %) Example of the Invention 1 17.7 −0.05 −0.54 0.032 0.256 Example of the Invention 2 15.6 0.19 1.92 0.01 0.08 Example of the Invention 3 17.2 −0.036 −0.36 0.021 0.168 Example of the Invention 4 20.2 −0.045 −0.45 0.034 0.272 Example of the Invention 5 22.3 0.09 0.87 0.017 0.136 Example of the Invention 6 20.1 −0.03 −0.3 0.02 0.16 Example of the Invention 7 15.4 −0.10 −0.99 0.059 0.472 Example of the Invention 8 20.3 −0.05 −0.45 0.032 0.256 Example of the Invention 9 21.3 −0.04 −0.40 0.025 0.2 Example of the Invention 10 23.3 −0.03 −0.26 0.017 0.136 Example of the Invention 11 18.1 −0.04 −0.33 0.017 0.136 Comparative Example 12  9.3 0.16 1.59 0.022 0.176 Comparative Example 13 14.3 −0.04 −0.36 0.022 0.176 Comparative Example 14 12.9 0.28 2.76 0.02 0.16

TABLE 5 Cyclic corrosion Immersion Test in a Urea Aqueous Solution Test Adhereing Property of Brass Testing Corrosion Maximum Spreadability Example rate Solution Analysis (unit: mg/kg) Corrosion Depth of brazing No. (g · m⁻² · h⁻¹) Fe Cr Ni Cu (μm) metal Microstructure Example of the Invention 1 <0.001 0.30 0.12 <0.05 <0.05 849 Good Good Example of the Invention 2 <0.001 0.35 0.15 <0.05 <0.05 887 Bad Bad Example of the Invention 3 <0.001 0.32 0.13 <0.05 <0.05 878 Good Good Example of the Invention 4 <0.001 0.25 0.06 <0.05 <0.05 663 Good Good Example of the Invention 5 <0.001 0.15 <0.05 <0.05 <0.05 431 Bad Good Example of the Invention 6 <0.001 0.20 0.06 <0.05 <0.05 598 Good Good Example of the Invention 7 <0.001 0.37 0.16 <0.05 <0.05 865 Good Good Example of the Invention 8 <0.001 0.26 0.06 <0.05 <0.05 585 Good Good Example of the Invention 9 <0.001 0.21 <0.05 <0.05 <0.05 323 Good Good Example of the Invention 10 <0.001 0.08 <0.05 <0.05 <0.05 354 Good Good Example of the Invention 11 <0.001 0.29 0.10 <0.05 <0.05 813 Good Good Comparative Example 12 0.005 0.88 0.29 <0.05 <0.05 >1000 Bad Good Comparative Example 13 <0.001 0.40 0.16 <0.05 <0.05 >1000 Good Good Comparative Example 14 <0.001 0.42 0.17 <0.05 <0.05 >1000 Bad Good

(The Immersion Test in a Urea Aqueous Solution)

A test specimen having a width of 20 mm and a length of 40 mm was cut off from the cold rolled steel sheet, and was subjected to wet-polishing by emery paper down to 600-grit. Then, the test specimen was subjected to a thermal treatment at 700° C. in air for one second to simulate welding for obtaining a mock surface status of a welded heat-affected zone.

Next, corrosion tests were carried out in which the thermally-treated test specimens of Testing Examples 1 to 14 were immersed in a urea aqueous solution having a concentration of 30% at 60° C. for 144 hours. The ratio of the solution volume to the test specimen area was set to 3.6 ml·cm⁻² in accordance with the metal corrosion test in “an anti-freezing liquid” of JIS K 2234, and a special grade reagent was used for urea being used for the preparation of the urea aqueous solution. After the completion of the corrosion tests, the corrosion rate was measured by weighing the test specimen, and a solution analysis was carried out by ICPS. The analyzed elements were Fe, Cr, Ni, and Cu.

(The Cyclic Corrosion Test)

A test specimen having a width of 70 mm and a length of 150 mm was cut off from the cold rolled steel sheet, and were subjected to wet-polished by emery paper down to 320-grit. Then, the test specimen was subjected to a thermal treatment at 700° C. in air for one second to simulate welding for obtaining a mock surface status of a welded heat-affected zone.

Next, the edge faces and the rear surfaces of the thermally-treated test specimens of Testing Examples 1 to 14 were coated with sealing tapes, and repetitive wet-dry cycle tests were carried out under conditions shown in FIG. 3. After the completion of 180 cycles, the corrosion product was removed, and the corrosion depths in the corroded areas were measured by the depth of focus of a microscope method. Here, with regard to conditions which are not defined in this specification, the conditions prescribed in JASO M609-91 were applied.

(Brazeability)

In a similar manner to the above-described “adhering-and-spreading-out property of brazing metal”, the adhering-and-spreading-out property of brazing metal was measured. Then, the microstructures of the cross sections of the test specimens were observed. The number of crystal grains existing in the sheet depth direction was measured in a 20 mm-long range parallel to the rolling direction, and the brazeability was evaluated as good if two or more crystal grains existed in the sheet depth direction, and the brazeability was evaluated as bad if only one crystal grain existed.

As shown in Table 5 and FIG. 2, the steels of Testing Examples 1 to 11 showed the maximum corrosion depths of less than 1 mm in the cyclic corrosion tests; and therefore, the steels of Testing Examples 1 to 11 were evaluated as good in the corrosion resistance against salt damage. Furthermore, the steels of Testing Examples 1 to 11 showed the corrosion rates of less than 0.001 g·m⁻²·h⁻¹ in the immersion tests in the urea aqueous solution, and the amounts of Fe, Cr, Cu, and Ni in the solution after the tests fulfilled the standards of JIS K 2247-1. Therefore, the steels of Testing Examples 1 to 11 were evaluated as good in the corrosion resistance on the inside surface.

Among the steels, the steels of Testing Examples 1, 3, 4, 6, 7, 8, 9, 10, and 11 showed the value of C+N of 0.015 or more and fulfilled the Equations (IV) and (V) of the present invention. These steels were evaluated as good in the adhering-and-spreading-out property of brazing metal, and the coarsening of crystal grains was suppressed when being brazed. In addition, the steel of Testing Example 5 showed the value of C+N of 0.015 or more; however, this steel did not fulfill the Equations (IV) and (V) of the present invention. In this steel, the coarsening of crystal grains was suppressed; however, this steel was evaluated as bad in the adhering-and-spreading-out property of brazing metal.

In addition, the steel of Testing Example 2 showed the value of C+N of less than 0.015 and did not fulfill the Equations (IV) and (V) of the present invention. In this steel, the coarsening of crystal grains occurred remarkably, and this steel was evaluated as bad in the adhering-and-spreading-out property of brazing metal.

The steel of Testing Example 12 showed less than 10% in both of the amount of Cr and the effective amount of Cr. This steel showed a low corrosion rate of 0.005 g·m⁻²·h⁻¹ or less in the immersion test in the urea aqueous solution; however, the amounts of Fe and Cr in the solution after the test failed to fulfill the standards of JIS K 2247-1.

The steel of Testing Example 13 showed both of the amount of Cr and the effective amount of Cr outside the ranges of the present invention, and the steel of Testing Example 14 showed the effective amount of Cr outside the range of the present invention. These steels fulfilled the standards of JIS K 2247-1, and were evaluated as good in elution characteristics against the urea aqueous solution. However, these steels showed the maximum corrosion depths of 1 mm or more in the cyclic corrosion tests; and therefore, these steels had bad corrosion resistances against salt damage.

INDUSTRIAL APPLICABILITY

The ferrite stainless steel of the present invention is a preferred material for a device that reduces NO_(x) from exhaust gas by using urea water in an internal combustion engine, mainly in a diesel engine, and in particular, a device related to a urea SCR system for vehicles, and preferred for a tank being used when storing, producing, and transporting urea water. 

1. A ferrite stainless steel for use in producing a urea water tank, comprising: in terms of mass %, C: 0.05% or less; N: 0.05% or less; Si: 0.02 to 1.5%; Mn: 0.02 to 2%; Cr: 15 to 23%; and either one or both of Nb and Ti at a content within a range of 8(C+N) to 1% (herein, C and N represent the contents of C and N (expressed by mass %), respectively, and the numerical values shown in front of the atomic symbols represent constant numbers), with the remainder being iron and unavoidable impurities, wherein an effective amount of Cr expressed by any one of the following Equations (I), (II), and (III) is 15% or more (herein, the atomic symbols in Equations (I) to (III) represent the contents of the elements (expressed by mass %), respectively, and numerical values shown in front of the atomic symbols represent constant numbers), in the case where only Nb is contained, the effective amount of Cr=Cr+4Si−2Mn  (I) in the case where only Ti is contained, the effective amount of Cr=Cr+4Si−2Mn−10Ti  (II) in the case where both of Nb and Ti are contained, the effective amount of Cr=Cr+4Si−2Mn−(10Ti−3Nb)  (III).
 2. The ferrite stainless steel for use in producing a urea water tank according to claim 1, which further comprises, in terms of mass %, one or more selected from Mo: 3% or less, Ni: 3% or less, Cu: 3% or less, V: 3% or less, and W: 5% or less.
 3. The ferrite stainless steel for use in producing a urea water tank according to claim 1, which further comprises, in terms of mass %, one or more selected from Ca: 0.002% or less, Mg: 0.002% or less, and B: 0.005% or less.
 4. The ferrite stainless steel for use in producing a urea water tank according to claim 1, wherein a content of C+N is in a range of 0.015% or more.
 5. The ferrite stainless steel for use in producing a urea water tank according to claim 1, which further comprises, in terms of mass %, Al: 0.5% or less, wherein Equations (IV) and (V) are fulfilled (herein, atomic symbols in Equations (IV) and (V) represent contents of the elements (expressed by mass %), and the numerical values shown in front of the atomic symbols represent constant numbers). Ti−3N≦0.03  (IV) 10(Ti−3N)+Al≦0.5  (V) 